1. Field of the Invention
The present invention relates in general to power transistors operated in a switching mode, and more specifically, to a direct base drive circuit used to drive power transistors.
2. Description of the Prior Art
It has long been common practice to operate power transistors in a nonlinear mode as on-off switches. Commercial use for such transistor power switches has, for example, found successful application in power conversion systems.
Of key importance in the operation of transistor power switches in the base drive circuit, which drives the transistor into its "off" or "on" state. Base drive circuits, which are common in the prior art, are of two types: transformer coupling; and direct base drive, in which a transformless coupling is used.
The transformer coupling is limited to applications which only require operation over a limited or substantially fixed range of duty cycle. Duty cycle is defined as the fractional ratio of the time period during which the output power switch is "on", to the total time period for the repetitive on-off cycle. Since magnetic coupling devices require operation with alternations of equal volt-second wave form areas, a transformer coupled base drive circuit experiences saturation effects and consequently a collapse of drive signals at duty cycle extremes. This precludes application of transformer-coupled base drive circuits in, for example, "boost regulator" circuits which require a duty cycle operation of from zero to 80 percent.
High performance "transformerless" direct base drive circuits are known in the art and are not subject to duty cycle restrictions. They are, however, limited to applications which do not require high output power levels. Direct base drive circuits, which are known in the art, drive the power transistor from a normal voltage source. Such voltage source drive circuits produce unfavorably large "turn-on" switching losses at high output power levels, due to their inability to deliver and maintain a high level of base current with adequate fast rise time during the switch "on" event. The rate of rise of base current drive, which can be produced by a voltage source, is limited by circuit inductances and load current reaction resulting from the power transistor output current. This detrimental turn "on" switching loss is experienced for output power levels of 1 kilowatt or greater.